The present invention relates to liquid injection molding machines and more particularly the present invention relates to runnerless molds for liquid injection molding machines.
Silicone compositions are well known. One particular type of a silicone composition is known as an addition curing silicone composition. Addition curing silicone compositions in a broader sense comprise a vinyl containing diorganopolysiloxane polymer of a viscosity varying anywhere from 100-500,000 centipoises at 25.degree. C., a hydrogen containing polysiloxane having a viscosity from 5 to 1,000 centipoise at 25.degree. C. and a platinum catalyst. This composition in the presence of the platinum catalyst will crosslink to form a silicone elastomer.
There has been various modifications to this composition, for example, adding a vinyl containing resin to increase the strength of the cured composition without increasing undesirably the uncured viscosity of the composition. An important part of the technology of such compositions is the use of inhibitors. Thus, in the uncured state, such compositions are normally packaged with a vinyl containing siloxane or with a hydride polymer and the vinyl polysiloxane in one package such that no package contains the vinyl siloxane, the hydride and platinum catalyst. If all three ingredients are in the same package, the composition cures to a silicone elastomer. Such a composition is normally packaged in two components. With the use of inhibitors the composition has a shelf life of anywhere from several hours to six months or more. One type of a mild inhibitor is a methyl vinyl polysiloxane. This is an inhibitor mixed into the composition in the parts per million level and inhibits the composition such that it does not cure for short periods of time at room temperature but cures rapidly at elevated tempertures, that is temperatures of above 100.degree. C. A more effective inhibitor is allyl isocyanurate disclosed in a patent of Burger ad Hardman U.S. Pat. No. 3,882,083 which is hereby incorporated by reference. Such an inhibitor gives a slightly more extended shelf life to the mixed composition before it cures to a silicone elastomer. Another effective inhibitor that has been found is diallylmaleate. Diallylmaleate is disclosed in the patent of Eckberg U.S. Pat. No. 4,256,870 is hereby incorporated by reference. While the Ekberg inhibitor makes the composition with an extended shelf life of several days, nevertheless when the composition is heated at elevated temperatures at temperatures above 100.degree. C. it cures in a matter of minutes and even seconds. However, there has been developed even more effective inhibitors which allow the three ingredients to be mixed without the compositions curing. One effective inhibitor for instance is the hydroperoxide compound of William J. Bobear disclosed in U.S. Pat. No. 4,061,609 which is hereby incorporated by reference. An effective amount of this hydroperoxide compound makes the composition stable in the mixed state, that is with the three ingredients mixed together and whatever other ingredients are necessary without curing for periods of six months or more. The composition cures at an elevated temperature in a matter of minutes or even seconds to a silicone elastomer. By elevated temperatures it is meant temperatures above 100.degree. C.
It has been found that a properly inhibited addition curing composition can be very effective as a molding composition for liquid injection molding machines. Traditionally, liquid injection molding machines have been utilized with organic thermoplastic compositions. These thermoplastic compositions were taken at room temperature, force fed into the mold either by the reciprocating screw type of injection molding machine or the ram plunger injection molding machine, causing the plastic to be heated and melted and then the plastic being cooled in the mold to form a desired part. The organic plastic in the traditional injection molding machines was fed to the machine in pellets. The heat of the friction created by the plunger or ram, in the injection molding machine caused the plastic pellets to melt and forced the plastic liquid out of the nozzle into the mold. The mold was then cooled so that the part came out as a solid thermoplastic part. Thermoplastic parts are then utilized for whatever purposes it was desired.
Such organic plastic liquid injection molding machines have found wide use in industry. It was found highly desirable to utilize silicone compositions in such injection molding machines to produce silicone parts with good high and low temperature properties. Accordingly, various types of machines have been designed or modified to produce liquid injection molding machines for silicones. It should be noted that silicone compositions differ from the traditional organic plastic compositions in the way they function in the molding machine in that they are liquid when introduced into the ram or plunger of the molding machine and that they are solidified by heating the mold to temperatures above 100.degree. C. which causes the silicone composition to jell and cross-link to a silicone elastomer as distinguished from the cooling of the mold in the organic plastic compounds. One example of a reciprocating screw molding machine which was adapted for silicone compositions can be found in the patent application of A. A. Laghi, Ser. No. 159,262 filed on June 13, 1980. This application discloses a valve means adapted to a reciprocating screw molding machine for introducing silicone compositions into the screw of the molding machine and for preventing the back pressure in the liquid feed tanks from affecting the pressure in the screw plunger action on the silicone composition fed into the mold. Further, there was disclosed a a seal modification on the screw plunger to keep the composition from leaking out from the backside of the screw plunger. These modifications were necessary in order to adapt the typical reciprocating screw plunger liquid injection molding machine so that it could utilize liquid compositions to form molded parts. In addition, the mold was heated so that the silicone composition was cured to a silicone elastomer.
Another type of a liquid injection machine that was modified to accept liquid silicone compositions was the plunger or ram type of liquid injection molding machine as disclosed in the patent application of A. A. Laghi, Ser. No. 183,620 filed Sept. 2, 1980, now abandoned, which is hereby incorporated by reference. As this patent application discloses, there were made various modifications to a plunger or ram type of liquid injection molding machine so that it could utilize silicone compositions and such that the machine could feed even shots of composition into the silicone mold. Again, the mold was heated for silicone compositions. True runnerless molds are known for organic plastics; but until the present time, true runnerless molds were not known for silicone compositions. It should be noted that molds for silicone compositions work differently from molds for organic plastics in that in silicone compositions, the mold has to be heated to cure the part whereas in organic plastic compositions, the mold is cooled to cure the part. Accordingly, the waste associated with prior liquid injection molding machines was eliminated by the advent of the molds of the present invention.
It is an object of the present invention to provide a runnerless mold for silicone liquid molding composition.
It is an additional object of the present invention to provide a mold for silicone injection molding compositions which does not waste material and which does not need the additional expense of a finishing operation to remove unwanted cured compositions from the molded part.
It is still an additional object of the present invention to provide a method for making a molded part from a silicone injection molding composition utilizing a runnerless mold.
It is yet an additional object of the present invention to provide a unique cut-off or shut-off means for a mold utilized in a liquid injection molding machine which is suited for silicone molding compositions.